1. Field of the Invention
This invention relates generally to the field of radar transmitters and more particularly to short pulse generation and radiation and time domain electromagnetics.
2. Description of the Prior Art
Radar Systems have a greater range and range resolution when the transmitted signal has a high peak power and short pulse duration. Conventional designs use a Marx generator to develop these signals. However, these designs have a limited pulse repetition frequency and pulse duration, and provide reduced high peak power. However, modifications to the conventional Marx generator eliminate these deficiencies. Therefore, a detailed description of the Marx generator is in order to better understand the invention.
Avalanche transistors were designed into the transmitter system to accomplish this need for improved performance at a very low cost. Previous attempts to increase the pulse repetition frequency (prf) of the source have been limited by avalanche transistor dissipation and power supply drain. As an example, the Marx generator configuration consisting of avalanche transistor sources permits one to charge a bank of capacitors in parallel from a low battery voltage and then discharge them in series creating a high voltage pulse. The Marx generator configuration is described in a text written by Miller, et al, entitled, "Time domain Measurements in Electromagnetics", Chapter 4, pp. 100-101, Van Nostrand Reinhold Company, N.Y., 1986.
To significantly increase the prf of the source beyond what has been achieved by other investigators, it is important to recognize the sources of current drain and power dissipation in the system. At first inspection, it appears that the only source of current drain from the dc supply is due to the charging of the coupling capacitors Cn of the Marx generator. Note that in the Marx generator design the capacitors are charged in parallel, and that the dissipation limitations of the avalanche transistors (e.g., an RS 3500 or the equivalent) determine the maximum prf. It can be shown mathematically that the average current during the charge cycle is determined only by the capacitor Cn. The two charging resistors R should be made as small as possible to ensure recharging capacitor Cn quickly for achieving a high prf. Since these resistors directly load the short pulse that is produced during the avalanche mode (e.g., they are effectively in parallel with an avalanche diode and balun circuit) they cannot be made too low in value. For example, in a 12 stage avalanche transistor Marx generator, where R=6.8K, there are effectively 24-6.8K resistors in parallel constituting about a 280 ohm load or the driving point impedance of the balun when exciting an antenna which results in a reduction of the high voltage signal. The decrease in output voltage can only be compensated for by increasing the number of avalanche transistor stages which, in turn, further increases the loading resulting in a vanishing small gain.
Other investigators have generated only baseband or video pulses at repetition frequencies of 1 kHz or less using this type of generator.